Switch Lowering in GCC

    cmp  eax,40
    ja   <return 0>
    test eax,7
    jnz  <return 0>
    jmp  JumpTable[rax]

... unless the size of immediate parameters would have affected code size. In the example code however, all constants fit in an `imm8` parameter to `cmp`.

Perfect hashing for switch tables is useful mostly for large, sparse switches, but hot large, sparse switches are rare, and the closest examples I can think of (e.g., parse tables) are themselves likely generated from other tools, which could itself be changed in lieu of the compiler. It really doesn't come across as a useful optimization to try implementing in a compiler.

For example, my bug report that `+= 2` is broken has not been acknowledged (read?) by anyone for 3 years:

    const size_t n = 13; // triggers the issue
    #pragma omp parallel for
      for (uint32_t i = 0; i < n; i += 2) {  // counts PAST 13!

>The LLVM patch was rejected a decade ago

And what was the reasoning?

Each `jmp` to f0..fX is an optimisation of a `call` and a `ret`, and probably done in a separate pass after the switch-case code has already been expanded.

The `mov edi, esi` at the top of the function is also an optimisation, most likely done during the register allocation pass. This was only possible because all functions take the same value in the same register. Otherwise, there would probably have been also a `mov` before each of the calls.

It is still possible that a later pass arranges jump targets to avoid false sharing of their entries (addresses) in prediction mechanisms and such.